1. Field of the Invention
The field of the invention is that of solid state gyrolasers used for the production of inertial systems necessary for the navigation of certain types of vehicles, such as aircraft. Gyrolasers are optical rotation sensors based on the Sagnac effect. The Sagnac effect, which is well known to specialists in this technical field, is not described in detail here. It is simply recalled that, when two contra-rotating optical modes circulate in a ring laser having a rotational movement, their optical frequencies undergo a shift representative of the speed of rotation.
2. Description of the Prior Art
At present, commercially available gyrolasers use a gaseous mixture of helium and neon as an amplifying medium. This technology, which is difficult to implement, has a certain number of disadvantages. Also, at present substituting the gas amplifying medium with a solid state amplifying medium, such as, for example, a neodymium-yttrium-aluminium-garnet (Nd-YAG) crystal pumped by laser diodes is being considered. The feasibility of such gyrolasers has been successfully demonstrated. In this matter, reference will be made to the publications of S. Schwartz, G. Feugnet, P. Bouyer, E. Lariontsev, A. Aspect and J. P. Pocholle in Physics Review Letters 97, 093902 (2006) and of S. Schwartz, G. Feugnet, E. Lariontsev and J. P. Pocholle in Physics Review A 76, 023807 (2007). The inertial performance of such a device improves as its frequency response becomes more linear, that is to say a beat signal between the two contra-rotating modes whose frequency is proportional to the speed of rotation of the assembly is obtained over an operating range that is as wide as possible.
A first source of non-linearity of the frequency response in solid state lasers is related to the existence of population inversion system in the gain medium, induced by stimulated emission. It is established, in the above references, that the deviation Δf from the ideal frequency response through this gain system is given by the following equations (A):
      Δ    ⁢                  ⁢    f    =                              γ          ·          η                          4          ⁢          π          ⁢                                          ⁢                      T            1                    ⁢          Ω                    ⁢                          ⁢      with      ⁢                          ⁢              Ω                  2          ⁢          π                      =                            4          ⁢          A                          λ          ⁢                                          ⁢          L                    ·              θ        .                            Where γ represents the rate of loss of intensity per unit time;        η is the relative pump excess above the pumping threshold;        T1 is the response time of the population inversion;        A is the area written in the laser cavity;        λ is the mean wavelength of laser emission;        L is the length of the cavity;        {dot over (θ)} is the speed of rotation of the gyrolaser.        
Putting the amplifying medium into longitudinal vibration makes it possible to avoid considerably the effects of the population inversion system described above. Additional information on this technique is available on referring to the patent FR 06 07394 issued on 26 Sep. 2008. Nevertheless, the stability of the pumping rate η remains an important criterion for the performance of solid state gyrolasers.
Another phenomenon capable of degrading the inertial performance is due to parametric resonances able to arise in the gyrolaser, related to the combined effects of the inertia of the gain medium and the excitation of the laser at the beat frequency when the latter is in rotation. It is in fact well known and is described, for example, in the book by A. Siegman entitled Lasers, University Science Books, Mill Valley, Calif. (1986), that certain lasers, called class B lasers for which the population inversion response time is very high in comparison with the other characteristic times, which are the lifetime of the coherences and the characteristic damping time of the cavity, exhibit a resonant response phenomenon about a certain characteristic frequency called the relaxation frequency and referenced ωr/2π in the rest of the description. When the speed of rotation of the solid state gyrolaser is such that the beat frequency is equal to or very close to the relaxation frequency, the intensities of the modes emitted by the laser becomes very highly unstable, which prevents the observation of the beating and therefore strongly reduces the overall inertial performance. In particular, a diode-pumped solid state gyrolaser using an Nd-YAG amplifying medium is a class B laser and is therefore subjected to this phenomenon.